In many environments such as industrial environments, multiple pieces of equipment are utilized which may share energy or power hazards. Under these circumstances it is important for safety purposes to incorporate EMO (Emergency Off) initiated from any piece of equipment which will shut off all the equipment if a hazardous condition occurs. An example of a patented EMO feature is illustrated in US Patent Publication US 2009/0066502, published Mar. 12, 2009.
A possible straightforward design of EMO linkage utilizes N EMO switches each having N contacts, where N is the number of pieces of equipment in a test cell. An example of a test cell in a manufacturing environment might include: a puncher; a presser; a packager; and a handler. The various pieces of equipment in a test cell may cover a large physical area, but EMO connectivity must be maintained between all the pieces of equipment. FIG. 1 illustrates an example of the aforementioned EMO linkage design. In this example, four pieces of equipment 100, 105, 110, and 115 share hazard conditions. Each piece of equipment includes a 4-contact EMO switch 120. Four power loops 125, 130, 135, and 140 are connected through each EMO switch, and a local EMO control circuit 145 is included in each power loop. If the power loop is closed, the local shut-down features are not activated. However, when any one of the EMO switches 120 are depressed, contacts 150 are broken for each power loop, thereby opening all four power loops. In this case, local EMO control circuits 145 are activated, shutting down all four pieces of equipment.
A problem with this aforementioned straightforward design is the difficulty in changing the equipment configuration, e.g., adding a new piece of equipment. Each time new equipment is added, an extra link in the chain (i.e. the power loops) is added, and one more contact must be added to the EMO switches for each piece of equipment in the cell. This makes field retrofit difficult, and may require design modification.